There are previously known rust proofing processes for a cast-iron brake drum or brake disc rotor, especially for a road wheel mounting plane thereof. These rust proofing processes include: (a) the application of rust preventive oil; (b) the combined application of rust preventive oil and paint (by means of a spray gun or dipping); (c) zinc phosphate conversion treatment; and (d) dacrodized treatment.
The process (a) i.e. the application of rust preventive oil only provides low rust preventive performance so that the road wheel mounting plane is prone to rust after mounted in a vehicle, although the rust preventive oil can be applied with simple operation. In the process (b) i.e. the combined application of rust preventive oil and paint, the road wheel mounting plane is made unpainted by means of masking, and the rust preventive oil is applied to the unpainted road wheel mounting plane. It is because that, when the whole of the road wheel mounting plane is covered with a coating of paint in such a manner that the coating has a standard thickness of about 20 to 30 μm, and then, a road wheel is fastened to the road wheel mounting plane by the use of wheel nuts, the wheel nuts tend to work loose owing to the coating thickness. However, the process (b) includes some additional process steps, such as masking, and produces paint waste. Moreover, the road wheel mounting plane is prone to rust even in the process (b) for the same reason as in the process (a).
The process (c) i.e. the zinc phosphate conversion treatment can be performed as disclosed in Japanese Patent Publication No. 1-58372. Much rust preventive performance is not expected in the process (c), as the zinc phosphate conversion treatment is originally intended for surface preparation prior to paint application. The process (d) i.e. the dacrodized treatment includes a larger number of process steps than paint application, thereby resulting in high overall cost for rust prevention. Further, much consideration must be given to the environment because of chrome used in the dacrodized treatment.